1. Field of the Invention
The present invention relates to a hydraulic pressure generating device capable of pressurizing a fluid in two steps, having a high-pressure accumulator, a high-pressure pump for supplying the high-pressure accumulator with a high pressure fluid, a charging accumulator for charging the high-pressure pump with a relatively low-pressure fluid, and a charging pump for charging the charging accumulator.
2. Discussion of the Prior Art
A pressure generating device of the type indicated above is used for example, for an anti-skid or anti-lock hydraulic braking system for an automotive vehicle. The high-pressure pump is operated in an intermittent manner. Namely, the high-pressure pump is turned on when the pressure in the high-pressure accumulator has falled below a preset lower limit, and is turned off when the pressure in the high-pressure accumulator has been raised to a preset upper limit.
In the case where a pressure generating device including a high-pressure accumulator is used for an anti-skid hydraulic braking system for a motor vehicle, the high-pressure accumulator stores the working fluid having a high pressure in the neighborhood of 200 Kg/cm.sup.2. When it becomes necessary to control the pressures in brake cylinders, for preventing the vehicle wheels from locking, the fluid stored in the high-pressure accumulator is delivered to a pressure regulating device.
A similar pressure generating device is used for activating a hydraulic booster which is used in a braking system for a motor vehicle, for boosting an operating force applied to a brake pedal or other operating member, so that the boosted force is applied to a master cylinder. While the braking system is at rest, the high-pressure pump is operated to store the fluid under a high pressure in the high-pressure accumulator. When the braking system is activated, the pressurized fluid is delivered from the high-pressure accumulator to the booster. Further, a pressure generating device is used for a traction control device in the vehicle braking system, for preventing the wheels from slipping on the road surface due to excessive drive forces.
In some braking systems, it is desirable that a high-pressure conduit is freed from a high pressure while the high-pressure pump is at rest. For instance, where the high-pressure conduit of a hydraulic system includes a flexible rubber hose, it is preferred to reduce the period during which the rubber hose is subjected to a high pressure, in order to prevent deterioration of the hose and increase the life expectancy of the hose. It is also preferred that a pump motor for the high-pressure pump is protected against an initial high pumping load upon starting, due to a high fluid pressure in the high-pressure conduit connected to the high-pressure pump.
In the pressure generating device for an anti-skid braking system for a motor vehicle, for example, a flexible high-pressure hose is used for connecting the high-pressure pump and the high-pressure accumulator, in such a manner as to allow a relative movement between the accumulator and the pump. The high-pressure flexible hose is subjected to a relatively high pressure, while the pump is operating, and even while the pump is at rest. This leads to shorting the life expectancy of the hose. In the light of this inconvenience, the assignee of the present invention proposed a pressure generating device as disclosed in Laid-Open Publication No. 61-131501 (published on Aug. 16, 1986) of Japanese Patent Application, wherein the high-pressure hose is freed from a high pressure while the pump is at rest. More specifically, a check valve is disposed between the high-pressure hose and the high-pressure accumulator, for preventing a flow of the fluid out of the high-pressure accumulator. Further, between the check valve and the high-pressure pump, there is provided depressurizing means for releasing the pressure in the high-pressure hose when the pump is turned off.
However, the above solution was not offered for a pressure generating device equipped with a charging pump and a charging acculator in addition to a high-pressure pump and a high-pressure accumulator. The present invention was developed to provide pressure relief means for such a pressure generating device, by utilizing the charging accumulator which functions to charge the high-pressure pump.
The pressure generating device indicated above suffers from another drawback. Stated in greater detail, when the pressure in the high-pressure accumulator has been lowered below the preset lower limit, the high-pressure pump is turned on to charge the high-pressure accumulator with the fluid pressure of the nominal level. The lower limit is set at a relatively high level, e.g., 140 Kg/cm.sup.2, sufficiently high to activate the booster of the braking system, for example. Therefore, the high-pressure pump must be operated to push the pressurized fluid into the high-pressure accumulator, against the relatively high pressure in the accumulator, from the very start of each pumping cycle. In other words, the pump motor for driving the high-pressure pump must receive this relatively high initial pumping load, as well as a load due to inertia of the pump motor and the high-pressure pump, when the pump device is started. Accordingly, the pump motor must have a relatively large capacity, enough to provide a relatively high power upon starting.
A DC motor is widely used as the pump motor for the high-pressure pump. In this case, the starting current applied to the DC motor is extremely high, resulting in rapid wear of the motor brush, and shortened service life of the motor.
The high-pressure pump may be driven by an engine of an automotive vehicle or other drive source provided for driving another device or component, via a clutch for selective connection of the drive source to the high-pressure pump. In this case, the capacity of the clutch must be commensurate with the load applied to the drive source upon starting. Further, the operation of the high-pressure pump causes a relatively large variation in the load being applied to the drive source such as the vehicle engine.
The above inconveniences or drawbacks will be encountered even if the pressure generating device is used for devices or systems other than a hydraulic braking system for automobiles.